The possibility of making stacks of thin layers of semiconductor materials that are of different natures and thus of different properties, lies behind numerous optoelectronic and microelectronic components, some of which are already being produced on an industrial scale. In most cases, such structures are built up from materials possessing lattice parameters that are equal or very similar (relative difference a few parts per thousand). This restriction is associated with a physical limitation that is often described as being the critical thickness for a material E of parameter a.sub.e grown epitaxially on a substrate material S of parameter a.sub.s, and which depends on the relative lattice mismatch (a.sub.e -a.sub.s)/a.sub.s between the two materials, and on implementation conditions.
At less than the critical thickness, the material E grows in two dimensional and stressed manner: its lattice parameter is subjected elastically to tetragonal deformation so as to be equal to that of the substrate in the plane of the layers. The layer of material E is said to be pseudomorphic. Pseudomorphic materials are in widespread use at present.
Above the critical thickness, either dislocation generation is observed in the interface plane, or the two-dimensional layer is observed to transform into islands of material. Both phenomena enable material E to relax.
In both cases, continued growth of material E will give rise to dislocations. These dislocations will pass through the thickness of the material E to find a free surface at which to terminate. The material E is thus degraded. When the material E has returned to its natural lattice parameter, the layer is said to be metamorphic. In order to obtain metamorphic layers of good quality, two kinds of approach can be found in the prior art.
A first approach consists in using intermediate buffer layers which serve to accumulate defects and to preserve the upper layers.
Such buffers may, for example, be stressed super-lattices (T. Won, S. Agarwala and H. Morkoc, Appl. Phys. Lett. 53 (1988), 2311) or graded composition layers (G. H. Olsen, M. S. Abrahams, C. J. Buiocchi and T. J. Zamerowski, J. Appl. Phys. 46 (1975), 1643, and J. C. Harmand, T. Matsuno and K. Inoue, Jap. J. Appl. Phys. 28 (1989), L1101), or indeed layers grown epitaxially at very low temperature (T. Ueda, S. Onozawa, M. Akiyama and M. Sakuta, J. Cryst. Growth 93 (1988), 517). They are generally more than one micron thick.
One case that has been studied extensively in the prior art is that of growing GaAs epitaxially on Si.
The use of thick buffers presents the drawback of buffer volume, given that the purpose of the buffer is to collect structural defects and not to have an active role in the operation of the component that is to be made subsequently. This "dead" volume greatly limits the possibilities offered by associating materials having lattice parameters that are very different since it does not enable two materials to be close enough together on either side of the buffer.
A more recent approach consists in using epitaxial adhesive between the two materials.
Each of the layers to be juxtaposed is then grown epitaxially on a substrate with a matching lattice. Their surfaces are put into contact with pressure being applied, and it is ensured that atomic bonds are formed by heating under a controlled atmosphere of hydrogen or nitrogen. Advantageous results have been obtained with this technique, with mismatch defects remaining confined close to the adhesive interface without spreading through the volume of the materials (G. Patriarche, F. Jeannes, F. Glas and J. L. Oudar, Proceedings of the 9th Int. Conf. on Microscopy of Semiconducting Materials, Oxford, 1995). This technique is presently being developed for making vertical cavity semiconductor lasers. Active layers that match InP are applied, for example, on mirrors that have been grown epitaxially on GaAs (D. I. Babic, K. Streubel, R. P. Mirin, N. M. Margalit, J. E. Bowers, E. L. Hu, D. E. Mars, L. Yang, K. Carey, Phys. Tech. Lett. (1995)). GaAs has been applied to Si (Y. H Yo, R. Bhat, D. M. Hwang, C. Chua and C. H. Lin, Appl. Phys. Lett. 62 (1993), 1038) or indeed InP has been applied to Si (K. Mori, K. Tobutome, K. Nishi and S. Sugou, Electron. Lett. 56 (1990), 737) using this technique.
In the context of epitaxial adhesion, work has so far been done on samples of very small surface area, of the order of 1 cm.sup.2. There is no guarantee that such a method will continue to be effective for larger areas. Also, that technique requires two separate epitaxial procedures on respective substrates, followed by said adhesion, and selective removal of one of the substrates. If it is desired to use such adhesion on a plurality of layers, the method becomes cumbersome. Finally, the presence of impurities such as O, C, and Si, at high concentration is probable at the adhesion interface, since, prior to adhesion, both surfaces are covered in a native oxide.